Our invention relates to a transmission control system of the kind shown in copending patent application Ser. No. 08/066,826, filed May 25, 1993, by Frank W. Timte and Ralph C. Bolz. That copending patent application, which is entitled "AUTOMATIC TRANSMISSION CONTROL SYSTEM", is assigned to the assignee of our invention.
Transmission gearing mechanism capable of being controlled by the control system of our invention may be seen by referring to U.S. Pat. No. 4,938,097.
Our invention includes features that are common to the control system described in U.S. Pat. No. 5,157,608, issued to B. G. Sankpal, J. A. Daubenmier, L. J. Kurdziel, J. F. Carnago, W. Lewis, Jr., and R. S. Williams. The '608 patent also is assigned to the assignee of our invention.
Each of the transmission structures disclosed in the reference patents and in the copending patent application mentioned above has four-speed ratio transmission gearing in which the high speed ratio is an overdrive. Each describes also gearing having a single reverse ratio. When a shift is made from the third ratio, which is a direct-drive ratio, to the next underdrive ratio, which is the second speed ratio, it is necessary to disengage a clutch and to engage a brake in synchronous fashion.
Both the clutch and the brake are friction elements. Each is operated by a fluid pressure operated servo. Thus, the downshift event requires a synchronous disengagement of a clutch and engagement of a brake. Because of differences in temperature and in friction coefficient due to wear, and because inertia forces differ depending on the speed at which a 3-2 downshift occurs, calibration of the 3-2 downshift to achieve optimum smoothness presents a design challenge. Even if the downshift can be properly calibrated for a given set of operating conditions, that calibration may not be appropriate at a later time when operating conditions are dissimilar.